Decremental conduction in the posterior and anterior AV nodal inputs

Effects of temperature on transjunctional voltage-dependent gating kinetics in Cx45 and Cx40 gap junction channels

Gap junctions (GJs) are intercellular channels directly linking neighbouring cells and are dodecamers of connexins. In the human heart, connexin40 (Cx40), Cx43, and Cx45 are expressed in different regions of the heart forming GJs ensuring rapid propagation of action potentials in the myocardium. Two of these connexins, Cx40 and Cx45, formed functional GJs with prominent transjunctional voltage-dependent gating (Vj-gating), which can be a mechanism to down regulate coupling conductance (Gj). It is not clear the effects of temperature on Vj-gating properties. We expressed Cx40 or Cx45 in N2A cells to study the Vj-gating extent, the kinetics of deactivation, and the recovery time course from deactivation at 22 °C, 28 °C, and 32 °C. Dynamic uncoupling between cell pairs were evaluated at different temperatures, junctional delays, and/or repeating frequencies. Cx40 or Cx45 GJs showed little changes in the extent of Vj-gating, but in both cases with a faster deactivation kinetics at high temperatures. The recovery from deactivation was faster at higher temperatures for Cx45 GJs, but not for Cx40 GJs. Cx45 GJs, but not Cx40 GJs, were dynamically uncoupled when sufficient junctional delays and/or repeating frequency in all tested temperatures. Gap junction specific dynamic uncoupling could play an important role in regulating action potential propagation speed in Cx45 enriched nodal cells in the heart.

Slow conduction through an arc of block: A basis for arrhythmia formation postmyocardial infarction

INTRODUCTION

The electrophysiologic basis for characteristic rate-dependent, constant-late-coupled (390 + 54 milliseconds) premature ventricular beats (PVBs) present 4-5 days following coronary artery occlusion were examined in 108 anesthetized dogs.

METHODS AND RESULTS

Fractionated/double potentials were observed in injured zone bipolar and composite electrograms at prolonged sinus cycle lengths (1,296 ± 396 milliseconds). At shorter cycle lengths, conduction of the delayed potential decremented, separating from the initial electrogram by a progressively prolonged isoelectric interval. With sufficient delay of the second potential following an isoelectric interval, a PVB was initiated. Both metastable and stable constant-coupled PVBs were associated with Wenckebach-like patterns of delayed activation following an isoelectric interval. Signal-averaging from the infarct border confirmed the presence of an isoelectric interval preceding the PVBs (N = 15). Pacing from the site of double potential formation accurately reproduced the surface ECG morphology (N = 15) of spontaneous PVBs. Closely-spaced epicardial mapping demonstrated delayed activation across an isoelectric interval representing "an arc of conduction block." Rate-dependent very slow antegrade conduction through a zone of apparent conduction block (N = 8) produced decremental activation delays until the delay was sufficient to excite epicardium distal to the original "arc of conduction block," resulting in PVB formation.

CONCLUSION

The present experiments demonstrate double potential formation and rate-dependent constant-coupled late PVB formation in infarcted dog hearts. Electrode recordings demonstrate a prolonged isoelectric period preceding PVB formation consistent with very slow conduction (<70 mm/s) across a line of apparent conduction block and may represent a new mechanism of PVB formation following myocardial infarction.

An easy-to-use technique to characterize cardiodynamics from first-return maps on ΔRR-intervals

Heart rate variability analysis using 24-h Holter monitoring is frequently performed to assess the cardiovascular status of a patient. The present retrospective study is based on the beat-to-beat interval variations or ΔRR, which offer a better view of the underlying structures governing the cardiodynamics than the common RR-intervals. By investigating data for three groups of adults (with normal sinus rhythm, congestive heart failure, and atrial fibrillation, respectively), we showed that the first-return maps built on ΔRR can be classified according to three structures: (i) a moderate central disk, (ii) a reduced central disk with well-defined segments, and (iii) a large triangular shape. These three very different structures can be distinguished by computing a Shannon entropy based on a symbolic dynamics and an asymmetry coefficient, here introduced to quantify the balance between accelerations and decelerations in the cardiac rhythm. The probability P111111 of successive heart beats without large beat-to-beat fluctuations allows to assess the regularity of the cardiodynamics. A characteristic time scale, corresponding to the partition inducing the largest Shannon entropy, was also introduced to quantify the ability of the heart to modulate its rhythm: it was significantly different for the three structures of first-return maps. A blind validation was performed to validate the technique.

Coexistent Types of Atrioventricular Nodal Re-Entrant Tachycardia: Implications for the Tachycardia Circuit

Background—

There is evidence that atypical fast–slow and typical atrioventricular nodal re-entrant tachycardia (AVNRT) do not use the same limb for fast conduction, but no data exist on patients who have presented with both typical and atypical forms of this tachycardia. We compared conduction intervals during typical and atypical AVNRT that occurred in the same patient.

Methods and Results—

In 20 of 1299 patients with AVNRT, both typical and atypical AVNRT were induced at electrophysiology study by pacing maneuvers and autonomic stimulation or occurred spontaneously. The mean age of the patients was 47.6±10.9 years (range, 32–75 years), and 11 patients (55%) were women. Tachycardia cycle lengths were 368.0±43.1 and 365.8±41.1 ms, and earliest retrograde activation was recorded at the coronary sinus ostium in 60% and 65% of patients with typical and atypical AVNRT, respectively. Thirteen patients (65%) displayed atypical AVNRT with fast–slow characteristics. By comparing conduction intervals during slow–fast and fast–slow AVNRT in the same patient, fast pathway conduction times during the 2 types of AVNRT were calculated. The mean difference between retrograde fast pathway conduction during slow–fast AVNRT and anterograde fast pathway conduction during fast–slow AVNRT was 41.8±39.7 ms and was significantly different when compared with the estimated between-measurement error (P=0.0055).

Conclusions—

Our data provide further evidence that typical slow–fast and atypical fast–slow AVNRT use different anatomic pathways for fast conduction.

Endovascular ablation of atrial fibrillation

Atrial fibrillation is the most common arrhythmia. The anesthetic considerations of endovascular ablation for the treatment of atrial fibrillation are reviewed.

Stable patterns of AH block arising from longitudinal dissociation and reentry within the superfused rabbit AV junction

INTRODUCTION

Multiple forms of antegrade AH Wenckebach block (WB) observed in 14 of 221 superfused rabbit AV junctions.

METHODS

Bipolar and microelectrode recordings were used to examine the mechanism of multiple forms of AH WB.

RESULTS

Each of the 14 preparations demonstrated typical 3:2 and 2:1 AV block, but also demonstrated longitudinal dissociation within the slow pathway input (N = 11) or compact AV node (N = 3). A 3:2 WB in one pathway and 1:1 conduction in a parallel pathway summated to provide antegrade conduction of the third atrial beat. Retrograde conduction (reentry) blocked conduction of the fourth impulse (4:3 block). A 5:4 block was similarly observed, with summation providing for antegrade AH activation of the third and fourth atrial beats. Retrograde activation observed with the fourth beat, terminated antegrade conduction of the fifth beat.

CONCLUSIONS

The studies demonstrate multiple AH WB patterns consistent with rate-dependent longitudinal dissociation, summation of dissociated AV conduction pathways, and retrograde conduction block within the AV junction.

The atrioventricular nodal reentrant tachycardia circuit: A proposal

Several models of the atrioventricular nodal reentrant tachycardia circuit have been proposed. Recently, there has been experimental and clinical electrophysiology evidence that the right and left inferior extensions of the human atriventricular node and the atrionodal inputs they facilitate may provide the anatomic substrate of the slow pathway. Inferior nodal extensions appear to constitute a necessary limb of the tachycardia circuit in all forms of atrioventricular nodal reentrant tachycardia and represent the ablation target for all forms of this arrhythmia. Anatomic variations of multiple atrionodal inputs via atrial transitional cells may create the conditions for tachycardia inducibility and differing patterns of retrograde atrial activation. In the present article, we summarize the available evidence and propose a comprehensive model of the tachycardia circuit for all forms of atrioventricular nodal reentrant tachycardia based on the concept of atrionodal inputs.

Delineation of AV Conduction Pathways by Selective Surgical Transection: Effects on Antegrade and Retrograde Transmission

INTRODUCTION

The role for transitional cells as determinants of AH and HA conduction was examined in the superfused rabbit AV junction.

METHODS

Bipolar electrodes and microelectrodes were used to record antegrade A-H and retrograde H-A activation, before and after transection of the transitional cell input to the compact AV node.

RESULTS

During pacing from the high right atrium, inferior to the coronary sinus os, beneath the fossa ovalis, or on the anterior limbus, AV Wenckebach block (WB) was mediated by identical transitional cells grouped in close apposition to the compact AV node. Paced WB cycle lengths were shorter from the high right atrium (196+/-12 msec) and inferior to the coronary sinus os (195+/-8 msec) versus the fossa ovalis (217+/-9 msec) or anterior limbus (206+/-11 msec). With His bundle pacing, retrograde HA WB (211+/-17 msec) was observed within the N cell region within the compact AV node. After transection of posterior and superior transitional cell input to the compact AV node, the antegrade AH WB cycle length was prolonged (245+/-18 msec), with an increased WB incidence within the NH region (compact AV node)(5% to 41%; p=0.014). The incidence of retrograde HA WB determined within the NH region was increased (30% to 88%), with a decrease in the stimulus-fast pathway conduction time (98+/-7 to 49+/-6 msec; p<0.01).

CONCLUSIONS

The data demonstrate (1) a common transitional cell population determining AH WB, independent of atrial stimulation site, and (2) a plasticity of transitional cell-compact AV node connections, with rapid AH and HA conduction favored by removal of posterior/superior AV nodal input.